A: Not out of the box, but the third party apps that slurp down hotmail and yahoo mailboxes could probably expose it as a POP feed. Would take some hacking to do though, since Thunderbird doesn't implement the full range of MAPI that many of the windows versions of these adaptors use.

Q: Has it got any extension mechanism similar to FireFox XPI? I would be particularly interested in linking it to VbClassic applications.

A: Its extension mechanism is XPI (XPCOM actually, xpi is the file extension that means "XPCOM Interface"). There are a scant few extensions, and it might be instructive to grab some of them and tinker with them. Be sure to share any results that you can -- documentation is otherwise so scant that aspiring hackers need the examples.